IPSec without AH

Erik Norgaard norgaard at locolomo.org
Sun Jan 23 05:55:07 PST 2005

J65nko BSD wrote:
>>Due to the problems of IPSec with NAT I was thinking if it is posible to
>>setup IPSec without Authenticated Headers? Does anyone know of a howto?

> The AH (Authenticated Header) protocol cannot be used with NAT, NAT
> modifies the header of packets, while AH is supposed to protect that
> header from being modified. Another IPSEC protocol ESP (Encrypted
> Security Payload), both authenticates and encrypts, and thus has no
> problem with NAT traversal.

Thanks, AFAIK, ESP and AH are used in conjunction in IPSec, ESP for 
encrypting the packet payload, and AH for authentication. ESP in it self 
does not provide authentication, but only encrypts the payload - hence 
the names :-)

Since ESP only encrypts the payload, as you say, ESP has no problem with 
NAT, whereas AH appends a signed checksum of the header. And since NAT 
alters the header, verifying the AH fails.

Ofcourse, it requires access to the (public?) keys to create valid 
encrypted packets. Hence, if the public key is kept as a shared secret 
among the authorized users, one could assume that ESP packets are 

This is my idea, discard AH, rely on ESP and assume that anyone capable 
of producing decryptable packets must have access to the pre-shared 
secret "public" key and hence authorized.

AH would work, if both ends were NATaware, such that the rigth src/dst 
ip could be inserted in the header before checking. It just occured to 
me that maybe this could be done by adding yet another IP/IP tunnel?

Cheers, Erik
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